PROBLEM OF EVOLUTION CAULLEEY. 325 



represented in the eighteenth century. It is unnecessary to say that 

 I use the word evolution in its nineteenth-century sense, which is 

 sj^nonymous with transformism. It is evident then that all is far 

 from being clear in the present conception of transformism and that, 

 in consequence, an exposition of its various aspects and an effort to 

 coordinate them is not a useless thing in a course of lectures. Fur- 

 thermore a comprehensive glance at the principal questions which 

 we shall have to examine will make my meaning clear and will give 

 me the chance to indicate the general plan of the course. 



In spite of the contradictions to which I have just alluded, the 

 reality of transformism as an accomplished fact is no longer seri- 

 ously questioned. We can make the statement that, in the unani- 

 mous opinion of biologists, evolution — that is to say, the gradual 

 differentiation of organisms from common ancestral forms — is the 

 only rational and scientific explanation of the diversity of fossil 

 and living beings. All the known facts come easily under this 

 hypothesis. All morphology in its different aspects, comparative 

 anatomy, embryology, paleontology, verifies it. By virtue of this 

 same hypothesis these different branches of morphology have made 

 an enormous progress since Darwin's day. The significance of cer- 

 tain categories of facts, especially in the domain of embryology, may 

 have been exaggerated. Scientific men have certainly overworked 

 the idea that the development of the individual, or ontogeny, was 

 an abridged repetition of phylogeny — that is to say, of the several 

 states through which the species had passed — an idea which Haeckel 

 raised to the fundamental law of biogenesis and which a whole gen- 

 eration of naturalists accepted almost as a dogma. Without doubt 

 ontogeny, in certain cases shows incontestable traces of previous 

 states, and for that reason embryology furnishes us with palpable 

 proofs of evolution and with valuable information concerning the 

 affinities of gi'oups. But there can no longer be any question of 

 systematically regarding individual development as a repetition of 

 the history of the stock. This conclusion results from the very prog- 

 ress made under the inspiration received from this imaginary law, 

 the law of biogenesis. 



The first part of the course will be devoted then to the consid- 

 eration of the general data which morphology furnishes toward 

 the support of the idea of evolution. Thus we shall see what con- 

 ception comparative anatomy, embryology, and paleontology afford 

 us of the way in which evolution is brought about, and within 

 what limits we may hope to reconstruct it. Evolution is essen- 

 tially a process which belongs to the past and even to a past extraor- 

 dinarily distant. It is a reasonable supposition that evolution 

 is going on to-day, but let us remember that nothing authorizes us 

 to believe that what we may observe in the present epoch about 

 73839°— SM 1916 22 



